The piloting of a vehicle, such as an aircraft (and to a limited extent other advanced vehicles, such as tanks), is recognized as an extremely complex task because of the fact that an aircraft is free to move while exercising six degrees of freedom. In the case of an aircraft, a pilot must reconstruct all six of these variables into a single trajectory and relate them to the three-dimensional worlds seen outside the windscreen. Some idea of the complexity of this task may be achieved by comparing it to the driving of an automobile, which is free to move with only two orthogonal degrees of freedom, thus resulting in only one angular degree of freedom which is, in any case, tied to the orthogonal movement, as long as no skidding is experienced.
Display of vehicle instrument data in a head-up mode against the view through the windscreen may be achieved by either use of a helmet-mounted display or a so-called head-up display. In the case of the head-up display, this may be done by placing a dichroic reflector in front of the windscreen and mounting a cathode ray tube with appropriate optical elements in the instrument panel of the vehicle to generate an image which may be reflected by the dichroic reflector to be viewed by the pilot. The dichroic reflector reflects only the wavelength of the CRT while also stopping only that wavelength from being transmitted to the pilot in his view of the outside world.
Thus, because of the properties of the dichroic reflector, the pilot may simultaneously see through the dichroic reflector and thus have an unobstructed view through the windscreen. This system allows the addition of other functions, for example enhanced images can also be sent to the cathode ray tube, thus allowing the pilot to "see" during poor visibility conditions. Likewise, data with respect to the orientation of guns and the flight path of an aircraft could also be displayed by the system. Moreover, by appropriate selection of optical elements between the cathode ray tube and the dichroic reflector, the images reflected by the dichroic reflector can be projected to optical infinity. Thus, the pilot can focus his eyes on the instrumentation and other information while still focusing on the scene outside his windscreen which is effectively at infinity.
In spite of the many obvious advantages of this system, it also has a number of problems which preclude its widespread employment in, for example, the aviation industry. Firstly, few existing aircraft have sufficient space available on an already crowded instrument panel to allow for the retro-fitting of this system. Secondly, even in the case of employment in newly designed aircraft, the demands for instrument panel space are such that the space is difficult or sometimes impossible to provide. Finally, there is also some objection to placing a dichroic reflector in front of the windscreen of an aircraft.
In an attempt to provide a display system which does not have the disadvantages of the instrument pane dichroic reflector system described above, systems in which a television cathode ray tube is mounted on an aircraft pilot's helmet for viewing by the pilot have been proposed. A display of the pilot's line-of-sight may also be synthesized in such systems, in order that the aircraft guns may be directed to fire along the pilot's line-of-sight onto a desired target. The pilot's helmet is necessarily provided with a device which detects the orientation of the pilot's helmet and, thus, his line-of-sight. This allows the slaving of guns or other systems on board the aircraft to movements of the pilot's head.